At present, most air-source heat pump air conditioner units adopt time defrosting control (see FIG. 1 for the defrosting period), or the defrosting is controlled by measuring the surface temperature of the finned tube of outdoor heat exchanger. However, these two defrosting control methods are prone to cause frost-free defrosting in cold regions of north China, which is bound to result in energy waste. In addition, there's a defrosting control method based on the air pressure drop passing the finned tube. The defect of this method is that: once the finned tube of outdoor heat exchanger is jammed by dusts, false judgment is caused, which also results in waste.